ntile() throws ERROR when hashagg is false

Hi

ntile() throws ERROR when hashagg is false, test case given below.

postgres=# create table foo (a int, b int, c text);
CREATE TABLE
postgres=# insert into foo select i%20, i%30, to_char(i%12, 'FM0000') from generate_series(0, 36) i;
INSERT 0 37
postgres=# explain select ntile(a) OVER () from foo GROUP BY a;
                            QUERY PLAN                            
-------------------------------------------------------------------
 WindowAgg  (cost=25.00..29.50 rows=200 width=8)
   ->  HashAggregate  (cost=25.00..27.00 rows=200 width=4)
         Group Key: a
         ->  Seq Scan on foo  (cost=0.00..22.00 rows=1200 width=4)
(4 rows)

postgres=# select ntile(a) OVER () from foo GROUP BY a;
 ntile
-------
     1
     1
     2
     2
     3
     3
     4
     4
     5
     5
     6
     6
     7
     7
     8
     8
     9
     9
    10
    11
(20 rows)

postgres=# set enable_hashagg to false;
SET
postgres=# explain select ntile(a) OVER () from foo GROUP BY a;
                               QUERY PLAN                               
-------------------------------------------------------------------------
 WindowAgg  (cost=83.37..91.87 rows=200 width=8)
   ->  Group  (cost=83.37..89.37 rows=200 width=4)
         Group Key: a
         ->  Sort  (cost=83.37..86.37 rows=1200 width=4)
               Sort Key: a
               ->  Seq Scan on foo  (cost=0.00..22.00 rows=1200 width=4)
(6 rows)
postgres=# select ntile(a) OVER () from foo GROUP BY a;
ERROR:  argument of ntile must be greater than zero